[ovs-dev] [PATCH] FAQ: Document the necessity of ct(alg=ftp).

[Joe Stringer]

Automatic helper assignment was disabled in Linux 4.7 or later, in
upstream commit 3bb398d925ec ("netfilter: nf_ct_helper: disable
automatic helper assignment").

Signed-off-by: Joe Stringer <joe at ovn.org>
---
 Documentation/faq/openflow.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/faq/openflow.rst b/Documentation/faq/openflow.rst
index d31bbef96c81..632f8e7190da 100644
--- a/Documentation/faq/openflow.rst
+++ b/Documentation/faq/openflow.rst
@@ -535,3 +535,17 @@ Q: The "learn" action can't learn the action I want, can you improve it?
     - At least some of the features described in T. A. Hoff, "Extending Open
       vSwitch to Facilitate Creation of Stateful SDN Applications".
 
+Q: When using the "ct" action with FTP connections, it doesn't seem to matter
+if I set the "alg=ftp" parameter in the action. Is this required?
+
+    A: Before Linux 4.7, automatic helper assignment was enabled by default.
+    This means is that even if you do not specify ALGs, the traffic will be put
+    through that ALG. In such cases, it is possible to construct OpenFlow
+    tables using conntrack actions that are missing the FTP option, and the
+    conntrack action will still track that FTP connection and correlate its
+    sessions. When using kernels 4.7 or higher, or if the "nf_conntrack_helper"
+    sysctl is disabled, you should always specify the alg option for FTP
+    control connections.
+
+    For more context, see the blog post from the netfilter team:
+    http://www.netfilter.org/news.html#2012-04-03
-- 
2.10.2